In a vehicle, a differential unit is employed to allow right and left driving wheels to rotate at different speeds. A structure of the differential unit used generally will be explained briefly hereafter. A pair of side gears such as bevel gears is disposed in a differential case while being opposed to each other, and a pinion gear is interposed between those side gears. The pinion gear is rotatably mounted on a pinion pin arranged perpendicular to a rotational center axis of the side gears while connected to the differential case. Additionally, a ring gear is arranged around the differential case while meshing with an output gear of a transmission, and power of the transmission is transmitted to the differential unit through the ring gear. The right and left side gears are individually connected to a drive shaft, and power is transmitted to the driving wheels therethrough.
As described above, said differential unit is mainly composed of the gears to differentiate the rotational speeds of the right and left wheels while increasing a reduction ratio, and therefore subjected to a large torque during running. For this reason, sufficient lubrication is required for the differential unit. As described, the transaxle is formed by integrating the differential unit and the transmission having an oil pump. Therefore, the differential unit of the transaxle may be lubricated by oil discharged from the oil pump during propelling the vehicle by a prime mover such as an engine. For example, Japanese Patent Laid-Open No. 2007-113636 describes a differential device for vehicle in which lubricating oil pressurized by an oil pump is injected to an oil hole of a differential case. An oil passage for letting through the lubricating oil is surrounded by a separate plate and ribs formed on an inner side wall of the differential housing. The differential plate is arranged to divide into an agitating section where the lubricating oil agitated by a ring gear of the differential is temporarily reserved, and a reservoir section where the lubricating oil lifted by the ring gear is reserved. Owing to the separate plate thus formed, an amount of the lubrication oil will not be increased excessively, and an increment of a agitation resistance will be suppressed.
In the device taught by the Japanese Patent Laid-Open No. 2007-113636, the oil pump is driven by the engine during propulsion of the vehicle so that the differential unit can be lubricated forcibly by the lubrication oil discharged from the oil pump. Even when the vehicle is towed without activating the engine so that the differential unit is rotated by rotations of the driving wheels, the differential unit may also be lubricated ceaselessly by the lubricating oil lifted from the agitating section by the rotating ring gear to be delivered to the differential case through the reservoir section. By such splash lubrication, the lubricating oil is centrifugally scattered by the rotation of the ring gear and reaches higher portion in the differential housing with an increase in the rotational speed of the ring gear. In order to lubricate the remaining portion in the differential housing other than the differential gear unit comprising the ring gear and the side gears when the vehicle is towed, the transaxle has to be modified to apply the lubricating oil scattered by the rotation of the ring gear to the highest portion in the differential housing.
A temperature in the differential housing or a transmission housing integrated therewith is raised by heat resulting from friction between gear tooth surfaces or friction in bearings, and consequently an internal pressure thereof is raised. If the internal pressure of the housing is raised excessively, a sealing member may be damaged to cause oil leakage. In order to avoid such disadvantage, a breather is formed on a relatively higher place of the housing to provide a communication between an inner space of the housing and the outside thereby depressurizing the housing. In the foregoing transaxle, the breather is formed on a junctional region between the differential housing (or a transaxle case) and the transmission housing or in the vicinity thereof so as to release the internal pressure of each housing to the outside. Given that the lubricating oil adheres to the breather or the vicinity thereof, the lubrication oil may blow out of the breather together with the air. In order to prevent an intrusion of the lubrication oil into the breather, a labyrinthine structure may be arranged on an inner opening of the breather of the housing.
In the differential device taught by Japanese Patent Laid-Open No. 2007-113636 adapted to perform the splash lubrication, the housing may be lubricated sufficiently by scattering the lubricating oil to near the highest place of the housing. However, the lubricating oil would be delivered excessively to the vicinity of the breather. In this situation, the lubricating oil may blow out of the breather together with the internal air with a pressure rise in the housing.